XSearch: Explainable Code Search via Concept-to-Code Alignment

With the emergence of deep learning, semantic code search has been widely adopted in both academia and industry. These approaches embed natural-language queries and code snippets into a shared embedding space and retrieve results based on vector similarity. Despite their strong performance on benchmark datasets, they often suffer from poor explainability and generalization. Retrieved code may appear semantically similar yet miss critical functional requirements of the query, while providing no explanation of why the result was retrieved. Moreover, such failures become more severe under distribution shift, where models struggle to generalize to unseen benchmarks. In this work, we propose XSearch, an intrinsically explainable code search framework. Our key insight is that, by relying on global embedding similarity, all existing retrievers inherently take an inductive view. They learn statistical patterns, rather than truly understand the query's functional requirements. Therefore, we address the problem by reformulating code search as a deductive concept alignment problem. At a high level, XSearch (i) identifies functional concepts in the query and (ii) explicitly aligns them with corresponding code statements. This explain-then-predict design not only produces inherent concept-level explanations, but also mitigates shortcut learning that harms out-of-distribution generalization. We train an encoder with explicit concept-alignment objectives and perform retrieval through explicit matching between query concepts and code statements. Experiments show that, when trained on CodeSearchNet with a small model size (GraphCodeBERT with 125M parameters), XSearch improves performance on out-of-distribution benchmarks from 0.02 to 0.33 (15×) over eight state-of-the-art retrievers, and consistently outperforms both encoder-based and decoder-based baselines with up to 7B parameters. A controlled user study further demonstrates that concept-alignment explanations enable users to accept or reject retrieved results both faster and more accurately. The source code is publicly available at https://github.com/code-philia/Xsearch.